The basic operation and structure of land mobile radio systems is known. Land mobile radio systems typically comprise one or more radio subscriber units and one or more repeaters that transceive information via RF channels. These repeaters are further located at one or more sites such that coverage is provided for subscriber units over a wider geographic area than can be provided from a single repeater. As multiple repeaters at multiple sites permits increased system capacity through re-use of the same RF channels at geographically-separated repeaters, methods are known to re-use these channels.
As increased repeater site density, or smaller cells, permits greater re-use of facilities, methods are needed to permit the system to operate with increasingly closer repeater sites and smaller cells. Further, as the size of the cells is reduced, the likelihood that a moving subscriber unit can complete a communication session within the same cell is reduced. Thus, methods are needed to provide the ability for a subscriber unit to hand-off an in-progress communication session from one repeater to a neighboring repeater with minimal disruption.
In general, the effectiveness of such hand-off methods directly impact system performance. For instance, the hand-off must protect co-channel interference between a subscriber unit using a particular channel at one repeater site and other subscribers using the same channel at other sites. Further, the hand-off methods must detect movement of the subscriber unit as it travels away from the repeater where it is assigned a channel, or else interference will occur as the unit gets closer to a site where the same channel is in use.
Methods in which the system's fixed-end equipment (repeaters, repeater controllers, and multi-site controllers) coordinate and direct the hand-off process to moving subscriber units are known. Thus, the fixed-end equipment monitors the quality of the signal currently received from the subscriber unit on the subscriber's assigned channel. When the monitored signal falls below a prescribed quality level, the multi-site controller directs the signal monitoring to be performed at the subscriber's adjacent repeaters. If the monitored signal from one of these sites exceeds a prescribed quality level, the controller then directs and coordinates the hand-off of the subscriber from the current site to the selected adjacent site. Such a method is known as "fixed network equipment directed hand-off".
Fixed network equipment hand-off methods suffer from the inability of the hand-off decision maker, i.e., the multi-site controller, to know when the subscriber unit may be travelling away from its assigned repeater and thus getting closer to adjacent repeaters. This is because testing at adjacent sites does not even occur until the quality of the monitored signal from the assigned repeater falls below the threshold.
Fixed network equipment hand-off methods also suffer from the high utilization of control links between the multi-site controller and the various repeaters that is required to control the signal monitoring and receive the quality measurements simultaneously for all subscriber units. To improve the ability to detect movement of the subscriber unit towards adjacent repeaters, testing must be performed frequently, thus resulting in increased control traffic on these links. Further, as the spacing between repeaters is reduced to increase the site density and channel re-use, the frequency of signal testing must be correspondingly increased since subscriber units now travel more quickly from one repeater to another. Moreover, the corresponding increased work-load on the multi-site controller to process all hand-off decisions further limits the frequency of testing that can be performed.
Further, fixed network equipment hand-off methods also suffer from non-symmetric signal transmission between the repeaters and the subscriber unit. Thus, compared to subscriber units, the site repeaters use higher power transmission and from elevated antennas. Further, the mixture of low-power hand-held portable subscriber units and high-powered vehicular-mounted subscriber units results in the repeaters monitoring signals with large variations in quality.
Another problem is co-channel interference. For example, a vehicular-mounted subscriber unit can move a large distance from its assigned repeater before its signal quality triggers testing at adjacent repeaters. Thus, as it gets closer to an adjacent repeater where a low-power portable unit may be using the same channel, the higher-power vehicular unit interferes with the lower-power portable unit.
Moreover, the loading of control links transporting hand-off measurement and control messages between the multi-site controller and the repeaters, and the corresponding work-load of the multi-site controller to process all hand-off decisions, effectively limits the performance and repeater site density that may be achieved.
Methods are known for the subscriber unit to assist in monitoring signal quality from its assigned and the adjacent repeaters, and then sending quality measurements to the fixed network equipment. These methods, known as "subscriber-assisted hand-off", are generally used in digital radio systems using time division multiplexing ("TDM") of the messages. As is known, in TDM systems, each channel is divided into time frames, with each frame further divided into a fixed number of time slots. Thus, multiple subscribers can simultaneously be assigned the same channel, with different units assigned to fixed slots within the frame for receiving and transmitting.
In such subscriber-assisted hand-off methods, quality measurements made by the subscriber unit and transmitted to the fixed network equipment are utilized together with quality measurements made by the fixed network equipment, such that the multi-site controller can make more appropriate hand-off decisions. Monitoring adjacent repeaters by the subscribers reduces the need for increased frequency of monitoring and testing by the fixed equipment. This is because each subscriber can scan adjacent repeaters during TDM time slots in which the subscriber unit is not receiving or transmitting.
Further, as TDM systems typically encode voice signals into digital packets for transmission in time slots, monitoring of signals can employ quality metrics based on both received signal strength and decoded bit error rates. Hand-off methods that employ such measurements are known.
Subscriber assisted hand-off methods, however, suffer some of the same problems as fixed-network equipment hand-off methods. Because of the potential transmission differences between subscribers and repeaters, fixed-network equipment measurements must still be performed. As such, control link and multi-site controller loading are still limiting factors.
Methods that shift the total hand-off decision process to the subscriber units are also known. One method, Subscriber-Directed Hand-off, for example, eliminates the limiting factors on site density and multi-site controller and control link loading. This method, however, suffers a similar problem as fixed-network equipment hand-off, since transmissions are monitored by only one side of the communications. Thus, the transmissions are now monitored by the subscriber unit instead of the fixed-network equipment. While transmission quality is the sole criteria in the hand-off decision, even though that quality measurement may include both signal strength and bit error rate, differences will still exist between the subscriber to repeater transmission path and the repeater to subscriber transmission path, thus resulting in co-channel interference.
Thus, there is a need for an improved method for subscriber directed hand-off in a multi-site radio communications system.